Barack Obama has declared Donald Trump “unfit” to be the next US leader as he urged Republicans to stop endorsing the billionaire.

The President said the property mogul “keeps on proving” he is unqualified to lead America after he steps down in January.

“I think the Republican nominee is unfit to serve as president,” Obama said at the White House.

He added that Trump has shown he is "woefully unprepared to do this job" after his comments on military families as well as his handle on foreign affairs.

"I said that last week, and he keeps on proving it,” Mr Obama said.

US President Barack Obama speaks during the third night of the Democratic National Convention at the Wells Fargo Center in Philadelphia, Pennsylvania (Image: Getty)

“The notion that he would attack a Gold Star family that has made such extraordinary sacrifices to our country, the fact that he doesn't appear to have basic knowledge around critical issues in Europe and the Middle East and Asia means that he's woefully unprepared to do this job."

Mr Obama was referring to Trump’s attack on the parents of dead US Muslim soldier, US Army Captain Humayun Khan.

Read More

The Republican nominee had attacked the soldier's mother, Ghazala Khan, for remaining silent while on the podium at last week’s Democratic Convention saying she had been silenced by her husband on religious grounds.

She had in fact been too upset to speak.

Mr Obama also turned on Republicans like House Speaker Paul Ryan, who have endorsed the former reality show star.

“There has to come a point at which you say ‘enough’” the president declared.

In response Trump hit back.

(Image: Getty Images)

Taking to Facebook he wrote: "Obama-Clinton have single-handedly destabilised the Middle East, handed Iraq, Libya and Syria to ISIS, and allowed our personnel to be slaughtered at Benghazi.

“Then they put Iran on the path to nuclear weapons. "Then they allowed dozens of veterans to die waiting for medical care that never came.

“Hillary Clinton put the whole country at risk with her illegal email server, deleted evidence of her crime, and lied repeatedly about her conduct which endangered us all.

“She is reckless with her emails, reckless with regime change, and reckless with American lives. "Our nation has been humiliated abroad and compromised by radical Islam brought onto our shores. We need change now."